A conventional MOSFET generally includes a semiconductor substrate, such as silicon (Si), having a source, a drain, and a channel positioned between the source and drain. A gate structure, formed by a gate conductor layer and a gate insulator layer, may be disposed above the channel. The gate insulator layer is typically composed of an oxide material such as silicon dioxide (SiO2) and the gate conductor layer is typically composed of polycrystalline silicon (poly Si).
Current flows through the channel of the MOSFET when an electric field is applied thereto. The amount of current flowing through the channel is, generally, directly proportional to the mobility of the carriers in the channel. Increasing the mobility of the carriers in the channel therefore increases the amount of current flowing through the channel. Circuits which utilize high mobility MOSFETs are capable of faster operation.
One method for increasing the mobility of the carriers in the channel of a MOSFET is to mechanically stress the channel. There are a number of known methods for mechanically stressing the channel of a MOSFET. One known method utilizes a strained Si layer which is grown on a relaxed Sil-xGex substrate. In another known method, a contact etch stop layer is used to mechanically stress the channel of a MOSFET. More specifically, a tensile capping contact etch stop layer induces strain in the channel, and improves NMOS channel mobility. In yet another known method, a SilxGex strain layer is selectively grown in the source/drain regions of the MOSFET to generate uni-axial compressive strain in the PMOS channel.
There are several disadvantages associated with these known methods. The MOSFET utilizing the strained Si layer grown over the relaxed Sil-xGex substrate is costly to fabricate and may exhibit crystal defects and poor heat conduction. The contact etch stop layer method requires a high level of stress and stress control. The Sil-xGex strain layer selectively grown in the source/drain regions significantly increases the cost of the MOSFET and potentially, can create junction leakage.
Accordingly, an alternative method for mechanically stressing the channel of a MOSFET is desired.